I think it's somewhere in the realm of alternate reality in the same universe type comics bullshit, but it's definitely not a prequel in the narrative sense. It's a new story, not a previous chapter in the established one.

I'll take ToD as a good example, although Indy is proper Saturday Matinee story-telling stuff. Each episode is what it is and happens in isolation. The timeline is sort of irrelevant (let's ignore Crystal Skull).

Until the Terminator franchise continues past the point where the first Terminator and Kyle Reece are sent back, they're all prequels (arguably). That is the initiation point in time of the Terminator narrative.